Blender dev build question
A build considered as not stable is a version in developpement. They have new functionnalities/changes that still need to be tested, or contains known bug that will be fixed before official releases. 

An Experimental Build Branch, as said in the description are build with another compiler than the "usual" one. They should be stable (release 2.78) but you never now what can appen using a compiler that wasn't used for tests. 

If you have no reason to use one of them, use the official release